Feed system for broad band antenna



June 20, 1961 G. J. DOUNDOULAKIS EI'AL 2,989,748

FEED SYSTEM FOR BROAD BAND ANTENNA 4 Sheets-Sheet 1 Filed Oct. 22, 1956INVENTO s George J. Douu Quid/5 Inn Koun m BY M 111mm, a! 6 ATTORNE June20, 1961 G. J. DOUNDOULAKIS ETAL 2,939,748

FEED SYSTEM FOR BROAD BAND ANTENNA 4 Sheets-Sheet 2 Filed 001;. 22, 1956a 2 a 4 M; w w d m a & v w mam. A f m M 7 47/27 wm w 7 m. Wm 4 -DL MJune 20, 1961 DOULAKIS ET AL D ANTENNA INVENTORS Q'earye J: DaandoutakwIra Kamen BY I g', I,

ULAKIS ETAL D ANTENNA June 20, 1961 United States Patent 2,989,748 FEEDSYSTEM FOR BROAD BAND ANTENNA George J. Doundoulakis, Brooklyn, and IraKamen, New York, N.Y., assignors to General Bronze Corporation, Garden C1ty, N.Y., a corporation of New York Filed Oct. 22, 1956, Ser. No.617,554 4 Claims. (Cl. 343--781) This invention relates to a feed systemfor a parabolic reflector type antenna operable over a wide band offrequencies.

The invention is useful, for example, in connection with antennas usedin radio communications by scatter propagation, wherein the antenna isused for both trans- 1111581011 and reception on different frequenciesover a wide band, and even for simultaneous transmission and receptionon different frequencies within said band. In the ensuing description,the invention is described as specifically applied to an antennaintended for operation in the UHF band over an approximate range of 750to 950 megacycles (a wavelength of approximately 30 to 40 centimeters),although the invention is equally applicable to antennas designed forother frequency bands and for many other purposes, such asfrequency-scanned radar.

It will be immediately appreciated by anyone familiar with this art thatvery substantial problems are involved in achieving an acceptableimpedance match between the waveguide and the antenna for a low standingwave ratio throughout a band of frequencies of such breadth (the totalbandwidth of 200 megacycles being almost 25% of the center frequency of850 megacycles).

It is therefore among the objects of this invention to provide a feedsystem which is capable of providing an acceptable match to an antennathroughout a band of frequencies of the breadth indicated-a system whichis also basically simple mechanically, well-adapted for commercialmanufacture and for continuous, trouble-free use under field conditions.

In the drawings:

FIGURE 1 is a fragmentary perspective view of a parabolic reflector-typeUHF antenna embodying features of the present invention.

FIGURE 2 is a fragmentary plan view of one of the flanges on the horn ofthe antenna of FIGURE 1;

FIGURE 3 is a fragmentary longitudinal sectional view of the horn;

FIGURES 4 and 5 are transverse sectional views of the horn, takesrespectively on the lines 44 and 55 of FIGURE 3;

FIGURES 6, 7 and 8 are Smith chart plots of the adr mittance coordinatesof the feed system, as measured at various reference planes in the hornbefore and after addition of certain reactance elements, FIGURE 6showing the complete Smith chart, and FIGURES 7 and 8 showing only thecentral portion thereof, at an expanded scale.

As may be seen in FIGURE 1, the illustrative antenna of the presentinvention includes a parabolic reflector 20 which is fabricated of asuitable metal such as sheet steel, which is supported on a tower.Projecting forwardly from the center of the parabola 20 along its axisof symmetry is a rectangular waveguide 22, which is mechanicallyself-supporting.

The internal transverse dimensions of the waveguide are sufficient tocarry the particular frequencies involved. In an exemplary antennaadapted to carry frequencies within the band of 750 to 950 megacycles,the Waveguide suitably has a larger internal transverse dimension (itswidth or a dimension) of 9.75 inches. At its inner end, adjacent thesurface of the parabola 20, the waveguide suitably has a shorterinternal transverse dimension (its height or b dimension) of 4.875inches, which is linearly tapered to 3 inches at the outer end of thewaveguide. This gives the waveguide a cut-off wavelength of 2a=49.5centimeters, which corresponds to a frequency of approximately 600megacycles.

Fixed at the outer end of the waveguide 22 to serve as the compoundradiating element, is a horn assembly generally indicated 24.

This horn assembly is shown in greater detail in FIG- URES 2 through 5.As may best be seen in FIGURE 3, the horn assembly 24 includes at itsinput end a length of waveguide 26 which forms an effective continuationof the main portion 22 (FIGURE 1) of the waveguide, being similarlytapered in its shorter dimension and being fitted to the end of the mainwaveguide so as to form an internally smooth joint 27. The horn 24 isequipped with a flange 28 by which it is bolted to a similar flange onthe outer end of the main waveguide 22.

The outer end of the horn 24 is divided into upper and lower branches 30and 32 of equal size, these two branches being defined by a commonforward wall 34 and by rearward walls 36 and 38 which are inclined at anangle of 45 degrees relative to the longitudinal axis of the waveguide.In the region where the two branches 30 and 32 merge into the waveguide26, each of the two inner corners 40 is rounded in the shape of a rightcircular cylinder whose axis is parallel to the longer dimension of thewaveguide to give a smooth transition from the waveguide into the twobranches 30 and 32. The main body of the horn is suitably formed of castaluminum, with internal surfaces machined.

Secured to the end wall 34 adjoining the two shorter side walls of thewaveguide are a pair of separator members 42 which are of cuspateshapei.e., they are generally wedge-shaped with their peaked surfaces 41being concavely curved in the form of right circular cylindricalsurfaces coaxial with the opposing curved surfaces 40. Thus, theconcavely curved surfaces of the separators 42 are at all pointsequidistant from the convexly curved surfaces 40. The peaks 43 of theseparator members, which point rearwardly along the waveguide 26, bisectthe shorter dimension of the waveguide, the cancave surfaces 41 beingsubstantially tangential to each other and to the inner surface of theend wall 34 of the horn, although they are cut oif at each side, asshown at 42a to facilitate assembly of the horn by permitting movementof the separator members through the waveguide 26. From the foregoingdescription, it can be seen that the shorter dimension of each of thetwo branches is onehalf that of the outer end of the waveguide 26, or1%".

Secured to the two inclined rear faces 36 and 38 of the horn are a pairof flanges 44 having rectangular apertures 46 therein, these aperturesforming smooth continua: tions of the two branches 30 and 32 of thehorn. The apertured flanges with their divergent surfaces 48 and 50serve as the radiating elements by which the radio frequency energy isdirected against the face of the parabola 26 (FIGURE 1). The inner faceof each of the flanges 44 is recessed to receive a dielectric platewhich is used to seal the waveguide hermetically and permit it to bepressurized, for example with dehumidified air.

At each of the short sides of each of the apertures 46, as shown inFIGURE 3, the sidewalls of the waveguide are tapered inwardly toward theaperture for a short distance as shown at 47. This has the effect oflaterally broadening the beam of energy emitted from the aperture, andmakes possible a more uniform illumination of the parabola 20. To thesame end, the divergent surfaces 48 and 50 of the flanges, shown inFIGURE 2, are also cut away at their corners to leave inclined surfaces48a and 50a.

The apertures 46 are by nature somewhat capacitive in their admittancecharacteristic, despite the fact that the Patented June 20, 1961,

beveled surfaces 47 projecting inwardly from their shorter sidewallsintroduce a certain amount of inductive susceptance which to some extentcounteracts the inherent capacitive susceptance of the apertures 46.

The separators 54 suitably have a width of 1 /2 inches and are mountedflush against the shorter sidewalls of the waveguide. This leaves a freespace between the inner faces of the two separators of 6.75 inches.These separators not only introduce a certain amount of complexadmittance into the circuit, the principal component of which iscapacitive susceptance, but they also serve as supports for a pair ofinductive irises 56 which are mounted on each of the branches 30 and 32of the horn.

As best shown in FIGURE 2, these irises 56 extend from theconvexly-curved inner surfaces 40 of the horn to the concavely-curvedpeaked surfaces 41 of the separators 42, in a plane a coincident withthe axis of the adjacent curved surface 40.

As best shown in FIGURE 4, the outer edges of the plates which form theirises 56 are flush with the shorter sidewalls of the two branches ofthe horn In one illustrative system, the irises are formed of sheetaluminum 0.142 inch thick and extend transversely into the guide for adistance of 1% inches, leaving a space of 6.50 inches between theirinner edges. The irises 56 introduce inductive susceptance into thesystem. Their purpose, in general terms, is to invert the order of theadmittance coordinates of the feed system as measured in the plane ofthe irises.

This effect can best be understood by reference to the Smith chart plotsof admittance coordinates as set forth in FIGURES 6 and 7. FIGURE 6shows, in the area designated A, a plurality of admittance coordinatesmeasured at spaced frequencies over the entire frequency band in whichthe feed system is to be used, the measurement being made in the plane a(FIGURE 3) which the irises 56 will occupy, but before addition of theirises. The small circles in the group A, which represent the admittancecoordinates corresponding to the various frequencies, are marked toindicate the frequencies which they respectively represent, ranging from750 to 950 megacycles in steps of approximately 30 megacycles. As mayreadily be understood by those familiar with this type of chart, all ofthese admittance coordinates have capacitive susceptance components ofthe same order of magnitude, and the standing wave ratios, as measuredover this band of frequencies, are also of the same general order ofmagnitude. Thus, the admittance coordinates over the entire frequencyband are closely bunched. It is important that the irises 56 be locatedsufficiently close to the apertures 46 that the admittance coordinatesare not spread too widely apart-in other words, that their respectivevectors are substantially in phase.

As is well known, as the reference is moved back along the waveguide ina direction toward the generator and away from the load, the admittancecoordinates as measured at the various frequencies move in a clockwisedirection in a generally circular path centered about the point of thechart, representing a normalized admittance of 1 (that is, an admittanceequal to the characteristic admittance of the waveguide), with theradius of the circle being equal to the reflection coefiicient. Thecircles in the group generally designated B represent the admittancecoordinates as measured at an arbitrary reference plane b (FIGURE 3)which is spaced along the waveguide in the direction of the generatorthrough a distance from the irises 56 corresponding to approximatelyone-quarter wavelength.

The effect of shifting the reference plane is greater at the higherfrequencies than at the lower frequencies, because the distance throughwhich the plane is shifted is greater in proportion to the wavelengthsof the higher frequencies than those of the lower frequencies. Thus, theadmittance coordinates as measured at the higher frequencies moveclockwise through a greater angle than do the coordinates as measured asthe lower frequencies. Thus, as the reference plane is shifted to b, theadmittance coordinates become spaced apart in a generally curved line inwhich the high frequencies leadthat is, the coordinates as measured atthe higher frequencies are more advanced in a clockwise direction thanthe coordinates as measured at the lower frequencies.

The addition of the irises 56 and separators 54 (FIG- URE 2) has thecomposite effect of introducing into the feed system net inductivesusceptances of suificient magnitude to give all of the admittances, asmeasured at plane a over the entire frequency band, susceptancecomponents which are inductive rather than capacitive.

This Will shift the admittance coordinates, as measured at plane b, fromthe positions indicated at B in FIGURE 6, to the positions shown at B inthe expanded chart of FIGURE 7. As may be seen, this shift has theelfect of inverting the order of the respective coordinates so that thelow frequency coordinates now lead in the clockwise direction.

If the reference plane is shifted still further back along the waveguidein the direction of the generator, all of the coordinates will movegenerally arcuatcly in a clockwise direction about the point 0 on thechart, the high frequency coordinates moving through a greater anglethan the low frequency coordinates. Thus, a point can be found at whichthe higher frequencies will catch up" with the lower frequencies. Thiseffect is illustrated by the coordinates in the group designated C inFIGURE 7, which represent the admittance cordinates as measured at thereference plane 0 (FIGURE 3) over the frequency band.

The substantial coincidence of the several coordinates indicates thatthe susceptance components of all the coordinates are of approximatelythe same magnitude, and, as may be seen, all of the susceptancecomponents are positive in polarity, i.e. capacitive. It will also benoted that the group of coordinates c is centered about the circularline 62 corresponding to conductance components having a normalizedvalue of 1that is, a value equal to the characteristic conductance ofthe waveguide.

As may be seen in FIGURES 3 and 5, mounted in the waveguide at plane 0is an inductive iris assembly 58 comprising a pair of iris plates 58aand 58b mounted with their outer edges flush against the inner faces ofopposite short side walls of the waveguide 26. The iris 58 introducesinto the waveguide at plane c an inductive susceptance of a magnitudesufiicient to counteract the capacitive susceptance which is present atthat point before addition of the iris. In the illustrative systempreviously refered to, the two iris plates 58a and 58b are each formedof sheet aluminum inch thick and extend into the waveguide 26 a distanceof 1% inches, leaving a space of 7 /2 inches between their inner edges.

The effect of introducing this inductive susceptance, as illustrated inFIGURE 8, is to shift all of the admittance coordinates in acounterclockwise direction along the circular line 62 in the directionof the reference line 63 corresponding to zero susceptance. Theinductive iris 56 has a greater effect on the susceptances as measuredat the lower frequencies than at the higher frequencies. Thus, the lowerfrequencies tend to move farther in the direction of the base line thanthe higher frequencies. The inductive susceptance of the iris 58 is ofsufficient magnitude to bring all of the admittance coordinates intoapproximate alignment with the reference line 63. Therefore, all of theadmittances, as measured over the band of frequencies, are substantiallypure conductances, without any substantial susceptance components, andtheir conductance components have a normalized value of the order ofunity-that is to say, the admittance of the feed system as measured atplane C over the entire band of frequencies after addition of theinductive iris 58, is approximately equal to the characteristicadmittance of the waveguide. This match of the feed system admittance tothe charcteristic admittance of the waveguide gives the electricaleffect of a waveguide of infinite length, substantially eliminatingreflection and affording a standing wave ratio not substantially greaterthan unity; Thus, as may be seen in FIGURE 8, a circle drawn about theorigin corresponding to the characteristic admittance of the waveguide,with a radius corresponding to a standing wave ratio of 1.2, willenclose all of the admittance coordinates of the feed system, asmeasured over the entire band of frequencies.

In many antennas, particularly of the parabolic reflector type, problemsare introduced because of reflections from the parabola back into theapertures of the radiating horn. This reflected energy creates standingwaves on the line, and has an adverse effect on the coherence orintelligibility of the signal-for example, it might create a ghost on atelevision signal or garble a radio teletype signal. According to thepresent invention, this problem is overcome by the use of a pair ofreflector plates 64 and 66 (FIGURE 1). These plates are formed of sheetaluminum, with their front faces substantially planar for ease ofmanufacture, and they are cut to a circular shape. They are supported onthe parabola 20 by means of brackets 68 which support the plates so thatthe surface at the center of each plate is substantially normal to aline intersecting the focus of the parabola 20. Minute adjustments maybe made in such positions to achieve the maximum radiation of energyfrom the reflector plates 64 and 66 back into the apertures 46 of thehorn 24. The spacing of the reflector plates forwardly of the parabola20 is adjusted so that the energy reflected from the plate 60 back intothe apertures 46 is 180 out of phase with the integrated energyreflected by the parabola 20 back into the apertures 46, and the size ofthe plates 60 is so chosen that the amount of energy reflected from theplates 60 is equal to the amount of energy reflected from the parabola20 into the apertures 46.

In a typical antenna intended for use in the frequency range previouslyindicated, employing a parabolic reflector 30 in diameter, with a focusof 9' from the center of the front face of the parabola, the reflectorplates have diameters of 22%" and are spaced with their inner edges 11%from the axis of the parabola and 2 forward of the face of the parabola,and tipped inwardly so that their outer edges are 3% forward of theirinner edges.

Since the energy reflected from the plates 64 and 66 is equal andopposite to the energy reflected from the parabola 20 into the apertures46, it will exactly counteract the effect of the latter. Thus, the netreflection is zero, and the standing wave ratio of the system remainssubstantially unity.

It will thus be understood that the feed system disclosed provides anexcellent impedance match between the antenna and the waveguide over theentire range of frequencies for which the system is designed.

It will therefore be understood that the present invention hasaccomplished the aforementioned and other obvious desirable objectives.It should be emphasized, however, that the particular embodiment of theinvention which is shown and described herein is intended as merelyillustrative rather than as restrictive of the invention and thatvarious changes may be made in this embodiment in order to adapt it tovarying conditions of use, without departing from the scope of theinvention as defined by the appended claims.

We claim:

1. A feed system for an antenna operable over a broad band offrequencies, comprising a length of rectangular waveguide, at lease oneaperture therein for radiation of radio frequency energy, a firstreactance means electrically coupled to said waveguide at a point atwhich the admittance vectors of said feed system, as measured at aplurality of frequencies throughout said band, are of the same generalangularity and all have susceptance components ,of a polarity oppositeto that of said reactance means, the reactance of-said reactance.

means being of a magnitude sufficient to give all of said admittancevectors, as measured after addition of said reactance means, susceptancecomponents of the same polarity as that of said reactance means, andsecond reactance means associated with said waveguide at a point spacedfrom said first reactance means in a direc-' tion away from saidaperture such distance that the admittances of said feed system oversaid band of frequencies, as measured at said point after addition ofsaid first reactance means but before addition of said second reactancemeans, have normalized conductance components of the general order ofunity, the reactance of said second reactance means being suflicientsubstantially to counteract the susceptance components of the lattersaid admittances and render said admittances, as measured after additionof said second reactance means, substantially pure conductancs of anormalized value of the general order of unitary.

2. A feed system for an antenna operable over a broad of frequencies,comprising a length of rectangular waveguide, at least one aperturetherein for radiation of radio frequency energy, a rfirst inductancemeans coupled to said waveguide at a point sufliciently close to .saidaperture that the admittance vectors of said feed system, as measured ata plurality of frequencies throughout said band in the absence of saidfirst inductance means, all have capacitive components and are of thesame general order of angularity, the inductive susceptance of saidfirst inductance means being of a magnitude suflicient to render all ofsaid admittances inductive, as measured after addition of said firstinductance means, second inductance means associated with said waveguideat a point spaced from said first inductance means in the direction awayfrom said aperture a distance suflicient that the admittances of saidfeed system at said frequencies, as measured in the absence of saidsecond inductance means, have capacitive susceptance components, withnormalized conductance components of the general order of unity, theinductive susceptance of said second inductance means being of amagnitude suflicient substantially to counteract the capacitivecomponents of the latter said admittances, and render said admittances,as measured after addition of said second inductance means,substantially pure conductances of a normalized value of the generalorder of unity.

'3. A rear feed system for a parabolic reflector-type antenna operableover a broad band of frequencies, comprising a length of rectangularwaveguide, the output end of said waveguide being divided on its shorterdimension into two branches of equal size, a pair of rearwardly directedapertures, one in each of said branches, a first pair of inductiveirises one mounted in each of said branches, each of said inductiveirises being located at a point sufliciently close to the aperture inits respective branch that the admittances of said branch, as measuredat a plurality of frequencies throughout said band in the absence ofsaid first inductance means, all have capacitive components and are ofthe same general order of angularity, the inductive susceptance of saidiris being of a magnitude suflicient to render all of said admittancesinductive, as measured after addition of said iris, a third inductiveiris mounted in said waveguide at a point spaced from said first pair ofinductive irises in a direction away from said apertures a distancesuflicient that the admittances of said feed system at said frequencies,as measured at the latter said point in the absence of said thirdinductive iris, have capacitive susceptance components, with normalizedconductance components of the general order of unity, the inductivesusceptance of said third inductive iris being of a magnitude suflicientsubstantially to counteract the capacitive components of the latter saidadmittances and render said admittances, as measured after addition ofsaid third inductive iris, substantially pure conductances of anormalized value of the general order of unity.

4. A rear feed system for a parabolic reflector-type antenna operableover a broad band of frequencies, comprising a length of rectangularwaveguide, the output end of said waveguide being divided on its shorterdimension into two branches of equal size, a pair of rearwardly directedapertures, one in each of said branches, the inner wall of each branchbeing convexly curved in the shape of a right circular cylinder, a pairof cuspate separator members projecting from opposite short sidewalls ofsaid waveguide where said waveguide divides into said branches, with thepeaks of said separator members pointing in a direction back along saidwaveguide and substantially bisecting the short dimension of saidwaveguide, and with the peaked surfaces of said separator members beingconcavely curved about the same axis as said inner walls and being atall points substantially equidistant therefrom, a pair of inductive irisplates in each of said branches, said iris plates extending fromopposite short sidewalls of said branch between said separator membersand said inner wall along a plane substantially coincident with saidaxis, said irises and said separators being of such relative size as torender inductive the susceptance components of said admittances, asmeasured after addition of said irises and separators, another inductiveiris mounted in said waveguide at a point spaced from said inductiveiris plates in a direction away from said apertures sufficient that theadmittances of said feed system at said frequencies, as measured in theabsence of said other inductive iris, have capacitive susceptance components, with normalized conductive components of the general order ofunity, the inductive susceptance of said other inductive iris being of amagnitude suflicient substantially to counteract the capacitivecomponents of the latter said admittances and render said admittances,as measured after addition of said other inductive iris, substantiallypure conductances of a normalized value of the general order of unity.

References Cited in the file of this patent UNITED STATES PATENTS2,566,900 McArthur Sept. 4, 1951 2,607,010 K0ck Aug. 12, 1952 2,671,855Van Atta Mar. 9, 1954 2,729,817 Cornbleet Jan. 3, 1956 2,824,305Ohlmemacher Feb. 18, 1958 2,887,683 Dyke May 19, 1959 FOREIGN PATENTS601,280 Great Britain May 3, 1948 708,614 Great Britain May 5, 1954UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No;2,98%748 v June 20 1961 GeorgeJ. Doundoulakis et a1.

It is hereby certifiedthat error appears in the above numberedpatentrequiring correction and that the said Letters Patent should readas "corrected below.

Column 6 line 22 after "broad" insert band Signed and sealed this 10thday of April 1962,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents

